The DNA of the macronucleus in the ciliated protozoans, Stylonychia and Oxytricha occur in low molecular weightpeices about the size expected for s single genes. Micronuclear DNA has a very high molecular weight. Each piece of macronuclear DNA has an AT richer region at one end, and each piece binds a single RNA polymerase molecule. The course by which a macronucleus is formed from a michronucleus after conjugation is being studied. Initially polytene chromosomes are formed in the developing macronucleus. The polytene chromosomes are transected by membraneous partitions that form in all interbands, and the molecular weight of the DNA is thereby sharply reduced. Each band becomes enclosed in a membraneious vesivle in which DNA degradation takes place. The result is the generatio of gene-sized pieces of DNA, and these are then replicated many times to form the maute macronucleus. These facts suggest that the chromosome is composed largely (90%) of DNA that fomrs spacers between successive genes. We are currently testing this hypothesis by determining the location of macronuclear sequences in micronuclear DNA using denaturation mapping of and RNA polymerase binding to micronuclear DNAs. In addition we are characterizing the DNA of the macronucleus by several physical methods.